Recent advances in technology enable a substrate to be coated by condensing a vapor onto the substrate. Using the vapor deposition method, coatings may enhance or augment chemical, physical, or electromagnetic properties of the substrate. Depending on the properties sought, coatings may be formed of either the same material as the substrate or a different material, such as a corrosion resistant alloy.
A common method of forming a vapor uses an electron gun to focus a beam of electrons on the surface of an evaporant material, which often is a metal. The electron beam causes high localized temperatures on the surface, which vaporizes a portion of the metal evaporant. The metal vapor occupies a deposition chamber, which has a high concentration of the metal vapor because the chamber is evacuated of gases before vaporizing begins. The vapor condenses on articles placed within the deposition chamber. The condensed metal cools until it forms a solid coating on the article. The thickness of the coating is controlled by varying the operating conditions, such as vapor pressure, temperature, deposition time, and mechanical manipulation of the substrate. Vapor phase deposition methods utilizing an electron beam are currently in widespread use. A wide variety of articles, for example turbine components, may be coated by vapor deposition.
The turbine section and combustion section of a gas turbine are subjected to high temperatures. The temperatures of combustion gases entering the turbine often exceed the maximum operating rating of the alloys from which the turbomachinery components are formed. Turbomachinery components include, for example, turbine blades, vanes, and walls.
Because turbomachinery components are subjected to high temperatures, they typically are cooled to maintain their structural integrity. Air is diverted from the turbine compressor outlet to cool the turbine blades, vanes, and walls. However, using compressor air for cooling diminishes turbine efficiency because such air is unavailable to support combustion. Thus, it is desirable to use a minimal amount of compressor air to cool the turbomachinery components.
Generally, cooling is accomplished by passing a constant flow of air through internal passages within the component. To achieve constant cooling air flow throughout an airfoil, internal airfoil static pressure must exceed airfoil external pressure. If external pressure exceeds internal pressure, combustion gases flow into the airfoil interior, which interrupts the cooling air flow and may damage the airfoil by overheating.
Moreover, cooling air loses pressure as it passes through serpentine passages or small film holes common to most air-cooled airfoils. Therefore, fluid pressure losses must be minimized to maintain internal pressure higher than external pressure.
There exists a need for airfoils and other turbomachinery components that have internal cooling passages that provide low cooling air pressure loss.
Conventional methods of fabricating airfoils employ both a casting step and an extensive machining step. The casting step requires fabricating a mold and a core. The core is supported within the mold by core support members. Molten metal is poured into the mold around the core and the core support members. After the metal cools, the core and core support members are dissolved by applying a chemical solution. Dissolving the core support members forms holes within the airfoil casting. The cast airfoil is then machined into its final shape. The cooling passages within the airfoil are formed by the voids that remain in the casting after the core is dissolved. Thus, the geometry of the mold core determines the shape and size of the passages.
The conventional method of producing airfoil passages has numerous drawbacks, including the high cost of machining. Moreover, the fabrication of the mold core is especially complex because of the intricate shapes of the cooling passages.
Furthermore, because of inherent limitations of the casting process, the passages must be larger than optimum and set deeply beneath a thick airfoil wall. For example, high temperature steel alloy blades of commercial, large-scale combustion turbines often have wall thickness of 0.189 inches maximum to 0.070 inches minimum. These dimensions would be typical of row one and two turbine blades used on Westinghouse frame 501G. Unfortunately, large, deep passages are less efficient at transferring heat from the airfoil than airfoils with cooling passages located closer to the airfoil outer surface, as is well understood in the art. The lower heat transfer efficiency of large, deeply set passages requires more cooling air, which diminishes overall turbine efficiency. Attempting by conventional methods to produce smaller passages and thinner airfoil walls is expensive and may result in a prohibitively low yield rate.
There exists a need for a reliable method for forming airfoils and other turbomachinery components having numerous small cooling passages that are located very close to the airfoil surface, without the prohibitively low yield rate of conventional methods. There also exists a need for airfoils and other turbomachinery components that have such passages.